Exception in callback error

The same thing happened to me. i made simple Exception in callback: recognition App from follow post. https://library.vuforia.com/articles/Solution/How-To-Create-a-Simple-Cloud-Recognition-App-in-Unity but i'm having problem.i register 2 targets on my cloud detabase. 1.play on Unity with cloud target. 2.put on 1st target on front of camera. program find 1st target. 3D Cube object is shown. no problem at this moment 3.1st target is out of camera. 4.put on 2nd target. following error occurred. Exception in callback: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> UnityEngine.MissingReferenceException: The object of type 'ImageTargetBehaviour' has been destroyed but you are still trying to access it.

Your script should either check if it is null or you should not destroy the object.

is there any solution?